Travelling wave tubes



June 9, 1959 D. c. ROGERS ETAL 2,890,371

TRAVELLING WAVE TUBES Filed Aug. 10, 1954 4 Sheets-Sheet 1 6 Inventor! D. C. ROGERS F? F. C. BURKE Attorney June 9, 1959 D. c. ROGERS ETAL TRAVELLING WAVE TUBES Fnd Aug. 10, 1954 4 Sheets-Sheet 2 EmEtnEt Inventors D C. RO'GERS' R F. C.BURKE M2$ Attorney June 9, 1959 D. c. ROGERS ETAL TRAVELLING WAVE TUBES 4 Sheets-Sheet 3 Filed Aug. 10, 1954 Altorney June 9, 1959 D. c. ROGERS ETAL 2,890,371

TRAVELLING WAVE TUBES Filed Aug. 10, 1954 4 Sheots Sheet 4 Inventors D. C. ROG E R8 P. F. C.BURKE A Home y United Sts Patent TRAVELLING WAVE TUBES Douglas Cecil Rogers and Peter Francis Conway Burke,

Application August 10, 1954, Serial No. 448,816

Claims priority, application Great Britain October 14, 1953 5 Claims. (Cl. 315-35) The present invention relates to travelling wave tubes of the kind comprising, within a glass envelope, an electron gun, a helix of conducting material for guiding electromagnetic Waves in a slow mode of propagation to interchange energy with electrons projected from the said electron gun and means affording coupling between the said helix and an external waveguide.

Travelling wave tubes are normally used as amplifiers but can also be constructed to function as oscillators or modulators, mixers and the like. There must, therefore, in use be at least one wave feeder associated with the helix :and generally there will be both input and output wave couplings. This wave coupling is usually achieved by inserting the tube through a hollow waveguide, built-out :cavity or the like, and providing probe antenna means associated with the helix internally of the tube envelope for coupling between the waveguide or cavity and the helix. Means must therefore normally be provided for avoiding undesirable leakage of wave energy around the outside of the tube through the waveguide apertures receiving the travelling wave tube. Again, this is normally :achieved by means of a waveguide choke formed between parts secured respectively to the waveguide and internally of the tube envelope. A major constructional problem in travelling wave tubes is thus seen to be the mounting within the tube envelope of an electrode assembly which must not only be very carefully aligned with respect to :the electron beam path, but must also be accurately positioned so that the tube may be inserted in associated :apparatus with the wave coupling means correctly positioned with respect to the external apparatus. A further complication arises due to the fact that the electron beam is normally constrained to its desired path by means of a magnetic field and the construction of the tube must be such as to enable it to be aligned accurately with respect to the axis of the magnetic field.

in the conventional travelling wave tube heretofore used, the electron gun is housed within an envelope bulb portion similar to the envelope of a conventional radio receiving tube; the helix and waveguide coupling components are then contained within a long tubular envelope portion of smaller diameter than the said bulb portion. This long tubular envelope portion is normally closed at the end opposite the electron gun by an electron collector electrode. Alignment and positioning of the various electrodes has been dependent to a large extent, if not entirely, upon the accurate dimensioning of the glass envelope portions of the tube. Owing to the inherent difficulties of accurate glass-working, it is very desirable in the manufacture of all types of electric discharge tube to avoid, so far as is possible, the use of glass to determine the relative position of electrodes.

It is an object of the present invention to provide a travelling wave tube, in the manufacture of which the electrode assembly is positioned independently of any glass envelope, and in which, furthermore, the positioning of the tube with respect to external associated apparatus does not require the use of critically dimensioned tubing for the glass envelope, which latter, therefore, can be manufactured from ordinary commercial grade tubing.

The collector electrode must be able to dissipate most of the power in the electron beam, which is some 10 to 20 times the output radio-frequency power. For tubes giving an output power of several watts this requires that the collector be in intimate contact with a cooler of considerable size. It is an object of the present invention to provide a travelling wave tube in which the cooler is integral with the tube and does not need to be attached after the tube has been inserted in its circuit.

It is a further object of the invention to provide a more rigid mounting for an internal magnetic shield and pole piece around the gun than is possible, due to the weight of the iron involved, when using a conventional glass stern such as used in radio receiving tubes.

It is a still further object of the invention to provide a travelling wave amplifier tube incorporating various constructional improvements which our experience in this field has found to be desirable.

In accordance with one aspect of the present invention there is provided a travelling wave tube comprising a glass envelope and within said envelope a unitary assembly of an electron gun, a helix for guiding electromagnetic waves to interchange energy with electrons projected from the said gun and means for coupling the said helix to an external wave guide, a first annular metal end cap member hermetically sealed to said envelope, a second metal end cap member secured to the said electron gun and hermetically sealed to said first metal end cap member.

In accordance with another aspect the invention provides a travelling wave tube comprising a first unitary assembly which comprises an electron gun, a terminal plate having leads hermetically sealed therethrough for connection to the said electron gun, a helix for guiding electromagnetic waves to interchange energy with the electrons projected from the said electron gun, and means comprising an antenna probe and a Wave-guide choke sleeve member affording coupling between the helix and an external waveguide; a second unitary assembly which comprises an electron collector electrode and a metal end cap sealed to one end of the glass envelope; mechanical coupling means having metal members secured respectively to the said first assembly and to the other end of the glass envelope such that, with the first unitary assembly inserted in the said glass envelope, the two assemblies are hermetically united by the sealing together of the said metal coupling members.

In large measure the said first unitary assembly is :based upon an electron gun structure which combines, in a single element, the electron gun, a ferromagnetic pole piece and a waveguide sleeve member to which structure the remaining members of the electrode assembly are secured.

The invention will be described with reference ,to the accompanying drawings, in which:

Fig. 1 shows, for purposes of comparison, a view in partial section of a known type of travelling wave tube mounted in its associated apparatus;

Fig. 2 shows a perspective view of a travelling wave tube according to the present invention;

Fig. 3 shows the component electrodes of the tube of Fig. 1 laid out to approximate an exploded view of the assembly thereof;

Fig. 4 shows, in partial section, an enlarged view of the electron gun end of the tube of Fig. 2;

Fig. shows a longitudinal cross section through the tube of Fig. 2 and its associated apparatus; and

Fig. 6 shows a preferred modification to parts of the tube of Figs. 2 to 5.

In Fig. 1 the travelling wave tube 1 comprises an el ctron gun 2 housed within an envelope bulb portion 3, which is provided with a conventional valve base 4 carrying pins 5 and a locating spigot 6 for use with a conventional type of valve socket providing 11C. supply connection means for feeding the electron and suitably polarising the helix of the tube. The helix '7 is housed within a long glass tube 8 sealed on to the bulb 3, the envelope portion 8 being closed at the far end by an electron collectorelectrode 9, to which is secured a cooler 10, having cooling fins 11. The tube is inserted through opposed apertures arranged the wide sides of an-input hollow rectangular waveguide 12 and a similar output waveguide '13, both of which extend into shortcircuited matchingsections 14 and 15 respectively. The input and output waveguides 12 and 13, with their extensions, form part of a unitary assembly 16, which also comprises means for maintaining the required magnetic field along the axis of the travelling wave tube, and means for mounting the tube. The assembly consists of a set of coils 17, each wound upon a brass former 18. The formers 18, in the positions intermediate the ends of the assembly, are each supported between brass bushes 19, or between a bush 19 and a washer 20, pairs of the latter bounding the respective waveguides. At the ends of the assembly steel plates 21 and 22 receive the ends of the adjacent formers 18 and are clamped together by means of a squirrel ca e system of ferromagnetic tie-rods 23, of which two are shown in Fig. 1. The waveguides 12 and 13, with their extensions, are held in position by means of annular brass bosses 24 let in the apertures in the waveguide walls about the travelling wave tube 1, forming a seating for the washers 20. The end plate 21 carries an insulating washer 25 which forms a support for the travelling wave tube at the electron gun end. A mounting plate 26, secured to the end plate 22 by means of distance pieces 27 and bolts 28, receives and locates a flange 29 mounted on the base 4 of the travelling wave tube.

Within the travelling wave tube envelope the electron gun 2 is mounted inside a cylindrical ferromagnetic pole piece and anode member, which is the only part of the gun visible in the drawing, and carries at its forward end a mica washer 30. The gun is located in position in the bulb portion 3 of the glass envelope by means of this. washer 30, together with other mica washers not shown. A hollow metal tube 31, through which the electron beam is projected, is mounted on the mica washer 30, and carries at its further end an inner waveguide choke member 32, which is formed of a cylindrical skirt portion closed by a centrally apertured front portion flush with the wall of waveguide 12. The helix 7 is mounted within the envelope portion 8 by means of glass rods 33, which rest against the inner wall of envelope portion 8 and whose respective ends fit into suitable recesses in the waveguide choke member 32, at the electron gun end, and in a similar waveguide choke member 34 at the electron collector end of the tube. The helix is of uniform pitch throughout its length and approximately one turn at each end projects into the respective waveguides 12 or 13. The ends of the Wire of the helix are bent to project parallel to the axis of the tube and are joined to short tubular members which surround the electron beam and are secured to the respective inner waveguide choke members 32 and 34. The straight ends of the helix wire, together with the members 35, form antenna whose bases are virtually in contact with one waveguide wall, and which are connected at their high potential ends to the helix. The members 32 and 34, together with the surrounding annular bosses 24, form quarter-wave chokes at either end of the helix portion of the travelling wave tube; by reason of their capacitative coupling with the respective bosses 24, the face portions of the inner waveguide choke members 32 and 34 can be regarded as continuations within the travelling wave tube envelope of the adjacent waveguide walls. It is usual to provide some attenuation adjacent the helix by coating the inner surface of a portion of the glass envelope with resistive material. This is indicated in Fig. l in the central portion of the travel ling wave tube where the helix is not visible.

T he end plate 22 of the assembly 16 carries a central hollow cylinder 36 of ferromagnetic material which surrounds the electron gun 2 and projects beyond it towards the base of the travelling wave tube. This cylinder helps to provide a low impedance path for the magnetic flux from the coil 17 into the internalpole piece member of the electron gun and also greatly attenuates the penetration of magnetic field towards the rear of the gun, so that the cathode of the electron gun is very adequately shielded from the magnetic focussing field. The end plate 21 provides the other pole piece of the magnet system.

The arrangement of Fig. 1 has been rather fully described in order to make clear some of the constructional problems regarding location of elements to be solved in the manufacture of the travelling wave tube and, by contrast, to assist in the understanding of the present invention. It should be noted, first, that there is a complete electrode system housed within an envelope the elements of which system have to be properly aligned with corresponding elements of the assembly 16: thus the end face of the electron gun 2 must be accurately positioned with respect to the forward edge of the end plate 22; the front face of the waveguide choke member 32 must be aligned with the inner wall of the input wave guide 12, while the corresponding waveguide choke member 34 must have its face portion aligned with the inner wall of the output waveguide 13. Furthermore, the axial alignment of the travelling wave tube in the assembly, which is of vital importance (as an incorrectly focused electron beam may well burn out the helix), depends upon the accurate positioning of the flange 29 with respect to the internal axis of the electrode system. The bore of the tube portion 8 must be of accurate dimensions and must be accurately coaxial with the bore of the bulb portion 3 of the envelope.

' Again it will be observed that there is a very considerable axial length from the end plate 22 to the commencement of the helix over which the electron beam performs no useful work. This, previously unavoidable, additional length of the electron beam not only increases the size of the complete travelling wave apparatus but also calls for a corresponding extra expenditure of power in the magnetising coils 17. The cooler 10 "is not integral with the electron collector electrode, but is attached thereto after the travelling wave tube is inserted in the assembly 16. Finally, it will be observed that the massive electron gun structure 2, containing a large amount of ferromagnetic material is mounted within the envelope bulb portion 3 by conventional means which are not really mechanically suitable for other than light electrode systems, such as employed in ordinary radio receiving valves.

In Figs. 2 and 3, which are reproduced from photographs of an experimental tube according to the present invention, the completely assembled travelling wave tube is shown in Fig. 2 and the principal component parts of the electrode assembly laid out in Fig. 3 for comparison with Fig. 2. The travelling wave tube comprises a unitary electrode assembly 37 housed within a glass envelope 38, which is sealed at either end to ferromagnetic end caps 39 and 40 respectively. At one end the tube carries pins 41 and a locating spigot 42 for insertion into a conventional valve socket. At the other end a cooler 43 is secured as an integral member of the tube and carries cooling fins 44 and an aligning and locating boss 45. Referring now to Fig. 3, the end cap 40 comprises two main parts: a ring 46, which is sealed to one end of the envelope 38, and an inner member 47 which is a sliding fit within the ring 46 and carries a glass inset 48 in which are sealed the pins 41 and the locating spigot 42 An electron gun structure 49, which will be more fully described later, is housed, similarly to the electron gun 2 of Fig. 1, in a ferromagnetic pole piece member 50, which is in the form of a cylinder closed by a face portion 51 integral with the anode of the electron gun. The electron gun structure 49 is mounted on a thin cylindrical skirt member 52 which is secured to the end cap member 47. To the face portion of the gun structure are welded three ceramic rods 53, which, at their other ends, are secured to a waveguide choke inner sleeve member 54 of ferromagnetic material. A helix 55 is supported in an accurately in ternally dimensioned glass tube 56. This tube 56 is the only accurately bored glass member in the travelling wave tube. One of its ends is a tight fit and the other a good sliding fit over respective thin walled annular protrusions from the face portion 51 of the gun structure and from the waveguide choke member 54. (These protrusions are not visible in Figs. 2 and 3, that at the gun end being, in fact, part of an anode insert in the face portion 51, which insert is omitted in Fig. 3.) A further pair of waveguide choke inner sleeve members 57 and 58, which have no direct counterpart in the arrangement of Fig. 1, are secured respectively to the electron gun structure and to the waveguide choke member 54 by means of pairs of rods 59 and 60. (On later models of the travelling wave tube these rods, as will be described later, have been substituted by plated portions of the ceramic rods 53.) The helix supporting tube 56 is slit near either end to allow the wire of the helix 55 to be brought out without fear of distorting the helix and to be welded to respective antenna probe members 61, of which one is indicated in Fig. 3, but which is shown more clearly in Figs. 4 and 5. One probe member 61 is secured to the waveguide structure 49 and the other is secured to the waveguide choke member 54. An electron collector electrode 62, carrying a tubular extension 63, which engages with an insulating washer mounted in choke member 54, is secured to the end cap 39 and carries a further tubular extension 64 communicating with a copper exhaust tubulation 65, which, in Fig. 3, is shown joined to a member 66 for connection to an exhaust system during processing of the valve. After processing the tubulation 65 is squeezed off and the cooler 43 is secured over the extension 64. In the assembly of the valve, which will be described in fuller detail later, the end cap 39, with the collector electrode, is sealed to one end of the glass envelope 38, the cap portion 46 being sealed to the other end. The remaining members, except for the cooler 43, are assembled together, inserted into the envelope until the tube 63 engages in the member 54, and the end cap members 46 and 47 are finally sealed together with a metal to metal seal.

The constructional details of the electron gun end of the tube are shown more clearly in Fig. 4, which shows a longitudinal cross section through the electron gun and the surrounding tube envelope. Largely because of the various thicknesses of the materials involved, the various components are not shown strictly to scale. It should be pointed out that the electron gun construction employed is that forming the subject matter of the application of P. F. C. Burke Serial No. 448,850 filed August 10, 1954, now US. Patent 2,822,492, granted February 4, 1958. The pole piece member 50 of the electron gun structure 49 is a hollow soft iron cylinder, closed at one end by the face portion 51, which includes an anode 67,

which may be of copper or of other non-magnetic metal let in the face portion of the pole piece. An indirectly heated cathode 68 is secured by means of a ceramic washer 69 whose diameter is less than the bore of the pole piece 50, and a further ceramic washer 70. The base of the cathode projects through the washer 69 and is turned or spun over so as to be clamped between the two ceramic Washers. The upper surface of washer 70 is ground flat and seats against an annular shoulder 71 in the base of the pole piece cylinder. The diameter of the washer 7b is somewhat less than that of the surrounding pole piece, and its thickness is slightly greater than the recess of the pole piece in which it sits. A focussing electrode '72 surrounding the cathode is seated upon the ceramic washer 69, is centred in the pole piece by means of a mica centring washer 73 and is held in position by means of a plurality of tie-rods 74, which pass through clearance holes in the ceramic washers 69 and 70. A hollow metal cylinder 75 fits inside the assembly of rods 74 and is welded to them, so as to clamp together the assembly of cathode, focussing electrode, and ceramic washers 69 and 70. The cathode being centred with respect to the focussing electrode prior to being clamped in position. This cathode and focussing electrode assembly is secured to the pole piece 50 a further mica or resilient metal washer 76 which is held against the washer 7t? by means of eyelets 77 welded to pins 78, which pass through the washer 76 and are secured in the base of the pole piece 50. As mentioned above, the ceramic washer 70 is slightly thicker than the corresponding recess in the pole piece in which it seats, so that it is pressed against the shoulder 71 due to the resilience of the washer 76. The face 51 of the pole piece 50 is provided with seatings for end caps 79, shown more clearly in Fig. 3, which are welded to the ceramic rods 53. During the assembly of the travelling wave tube these end caps 79 are welded to the face 51 of the electron gun structure. This portion of the gun structure also carries one end of the helix supporting tube 56, which in Pig. 4 is shown seated over a thin resilient tube protruding from the anode member 67. Finally the face member 51 of the gun structure is shown in Fig. 4 carrying an antenna probe member 61 which is joined to the end of the helix 55. The probe member 61 is generally U-shaped so as to allow the helix to be brought up closer to the electron gun as discussed in the copending application of D. C. Rogers Serial No. 407,732 filed February 2, 1954. The drawing of Fig. 4 also shows the way in which the tube 51 is slotted at 80 to permit the end of the helix wire being brought out without risk of distortion of the end turns of the helix.

Towards its base the outer surface of the cylindrical pole piece 50 is reduced in diameter to receive the metal skirt 52. The glass terminal plate 48 let into the end cap portion 47 is sealed to an additional cup-shaped member 81 to which is secured a metal spinning 82 fitting over part of the cylindrical portion of the member 81 and engaging internally with the end cap member 46. The junction between the members 81 and 82 must provide a sound hermetic seal, the parts normally being hard soldered together before scaling in the glass insert 48. The lower end of the skirt 52 sits over the end of member 81 and butts against the spinning 82, being Welded in position to secure the end cap member 47 to the electron gun structure 49. To avoid heat losses and also conveniently to provide access for connection between the electron gun electrodes and the lead-out pins 41, the skirt member 52 is cut away as much as is consistent with its mechanical strength by means of a series of apertures 33, which are shown in proper proportion in Figs. 2 and 3. it is found that these apertures allow adequate access for Welding leads such as 84 to the pin 41.

As mentioned previously, the glass envelope 38 is sealed to the end cap member 46, in a manner to be described later, the seal being indicated at 85. The outer diameter of the end cap portion 46 is slightly greater than that of the envelope 38, the travelling wave tube being supported at one end in its circuit by the end cap. As has also been mentioned previously all the electrodes of the travelling wave tube except for the electron collector electrode are mounted from the gun structure 49 by means of the ceramic rods 53 so as to form a unitary assembly which may then be inserted into the glass envelope through the end cap member 46, which is finally sealed to the end cap portion by means of a flash braze. (By a flash braze we mean a brazing or welding operation which is rapidly completed before excessive heat is allowed to reach the glass insert 48.)

For comparison with the assembly of Fig. 1 there is shown in Fig. an assembly of the travelling wave tube 37 of Figs. 2, 3 and 4 in a montage 86 which is generally similar to the assembly 16 of Fig. 1, but modified to accommodate the'tube of the present invention; similar reference numerals will therefore be used as far as pos sible to identify corresponding parts of the two assemblies. One of the main dilferences between the arrange ment of Fig. 1 and that of Fig. 5 is that in embodiments of the present invention Lhe electron beam path from the exit of the electron gun to the entry into the helix has been very considerably reduced, and, in fact, the face of the electron gun is brought up flush with the inner surface of the input waveguide 12, instead of being some distance therefrom as in Fig. 1. The electron gun structure itself performs the function of the inner sleeve member 32 of Fig. l as well as forming a pole piece for the magnetic focussing system. This means that corresponding outer pole piece members must be brought right up to the input waveguide and that the outer pole piece members must also function as part of the waveguide choke system in place of the boss 24 surrounding member 32 in Fig. 1. In Fig. 5 in place of the aforementioned boss 24 a comparatively small brass collar 87 is let into the waveguide. A ferromagnetic cylinder88, which surrounds the electron gun structure and the base of the travelling wave tube, butts against the collar 87 and against the end plate 22 of the assembly. As the cylinder 88 forms part of a waveguide choke, its inner surface is copper-plated. It will be observed that the waveguide choke formed between the structure 49 and the member 88 is no longer a quarter wave length as was the corresponding choke of Fig. 1; its dimensions are, however, such as to provide adequate attenuation for electromagnetic waves tending to escape from the waveguide system past the electron gun. To ensure that the external pole piece system extends right up to the waveguide, an additional cylinder 89 of ferromagnetic material is provided. This is re ceased internally to clear the collar 87 and butts against the wall of waveguide 12 and its extension 14. At its other end the cylinder 89 is in magnetic contact with the end plate 22. At its forward end its outer surface is slightly recessed to receive a brass end plate 9?.) of an additional coil 91, which is considerably smaller than any of the coils 17, it being required only to compensate for the gap between the end plate 22 and the waveguide 12 and its extension 14. It will be observed that the external pole piecesystem does not end flush with the front face 51 of the electron gun structure 49, but is rearward thereof by the thickness of the waveguide wall. The slight ensuing non-uniformity of magnetic field is advantageous to the proper shaping of the magnetic lines of force where the electron beam leaves the electron gun and enters the magnetic field.

In place of the supporting plate 26 of Fig. 1 from which the travelling Wave tube 1 is positioned, suppor for the electron gun end of the tube is provided by ferromagnetic cylinder 88 in which the end cap i sliding fit.

At the other end of the travelling Wave tube we have shown in Fig. 5 an external pole picce arrangement gen erally similar to that at the input end of the tube. As the magnetic field conditions at this end are not so critical, a simpler construction can be used if desired. The wave-guide choke member 54 is of ferromagnetic material and provides the internal pole piece of the magnet the normal helix mode.

system. Similar" members 87, 89 and 94.! a coil 91 are shown, as at the electron gun end of the assembly, together with a cylindrical portion 92, forming part of the outer Waveguide choke member at this end, surrounding the internal member 54 and integral with the end plate As mentioned previously, the inner choke member 54 carries a probe antenna 61 and is secured to the ends of the ceramic rods 53, the arrangement being generally similar to that at the input end of the tube. Internally the member 54 car ies a mica locating washer it receives the end of the tube 63 projecting from the forward end of the electron collector electrode 62. The tube 63 is formed with a seating on its outer end surface to mate with the washer 93, and also carries a series of holes 94 to assist in evacuating the portion of the travelling wave tube enclosed by the helix supportin tube 56. On its other side, as previously mentioned, the electron collector electrode carries a tubular extension 64 in which is sealed the copper exhaust tubulation 65, here shown sealed off. At some convenient stage after the sealing-off operation, the cooler 43 is secured over the tube 64- and is made an integral part of the travelling tube, whereas in Fig. 1 the cooler is attached after the travelling wave tube is inserted into its associated apparatus. The cooler carries a projection 95 which engages with a metal boss 96 let into a supporting platform 97, which is mounted by means of insulating supports 98 from the end plate 21. The projection 95 carries a key-way 99, which is visible in Fig. 3, in which a peg 100, let into the boss 96, engages to ensure the correct angular location of the travelling wave tube. The projection 95 is also threaded internally to engage a screw N1 having a knolled head 102 of insulating material. By tightening the screw 101 the projection 95 can be drawn up to the limit of the recess in the boss 96 and the tube firmly fastened in position.

Due to the larger diameter of the travelling wave tube of the present invention compared with the tube 1 of Fig. 1, provision must be made to prevent leakage of electromagnetic energy along the outside of the helix from the waveguide apertures. In Fig. 1 the intermediate bosses 24 immediately surrounding the helix are of sufficiently small internal diameter to prevent the propagation electromagnetic Waves in any mode other than The waveguide apertures in 'Fig. 5, however, are much larger and some choke provision intermediate the ends of the tube in addition to those already mentioned must be provided. To this end brass bushes 103 are let into the waveguides immediately surrounding the travelling wave tube and the travelling wave tube assembly comprises the additional inner wave guide choke members 57 and 58, previously mentioned, which are generally similar to the choke members 32 and 34 of Fig. 1, comprising a disc portion electrically continuous with the waveguide wall and a skirt portion projecting towards the centre of the tube. A coating of resistive material 1 55, such as colloidal graphite or an evaporated metal is preferably provided on the inside of the envelope 38 to further attenuate electromagnetic waves which may travel between output and input waveguides in modes such that there exist appreciable radio frequency fields at a distance from the helix. Such coating is not included in the tube of Fig. 2 but has been found desirable on later models.

Again because of the greater diameter of the travelling wave tube compared with that of Fig. 1, the short circuiting of the Waveguide sections 14 and 15 cannot conveniently be effected outside the tube envelope. In Fig. 5 short circuit terminations are, in fact, shown at 104 but effectively the actual termination of the waveguide 12 and its extension 14 is provided by the pair of shorting bars 59 secured between the electron gun structure and the sleeve member 57. Similarly the shorting bars 63 terminate the extension 15 of waveguide 13. The shorting bars also provide mechanical mounting means for the choke members 57 and 58. The ceramic rods 53 pass through clearance holes in these intermediate choke members.

In the assembly of the travelling wave tube the electron gun structure 49, the helix55 in its support tube 56, and the waveguide choke members 57, 58 which, however, are undersize and do not need to be very accurately aligned and the member 54 are assembled together with the ceramic rods 53 in a jig which ensures accurate alignment and positioning of the respective electrodes. The end caps 79 of the ceramicrods 53 are a loose fit in the respective recesses of the electron gun structure and waveguide 'choke' member 54. When the various components have been correctly positioned the flanges on the caps 79 are welded to the electron gun structure or to the waveguide choke member- 54 respectively. Since the distance apart of the internal pole piece members 50 and 54 is determined by the distance between the end caps on the ceramic rods it is necessary to have the helix supporting tube 56 slightly under length to allow for thermal expansion changes. This is done and the tube made a very tight fit on one of the annular projections holding it. This can conveniently be done by slitting one of these projections, as indicated at 106 in Fig. 4 and springing it out a little.

In more recent models of the travelling wave tube than that illustrated in Figs. 2 to 5, the shorting bars 59 and 60 have been eliminated and modifications have been made to the method of mounting the electrode system on the ceramic rods 53. These alternative arrangements are illustrated in Fig. 6 which shows part of the electron gun structure 49 and the adjacent waveguide choke member 57. In place of the end caps 79 of Figs. 2 to 5, the ceramic rods 53 have flanged sleeves 107 welded to plated areas adjacent the ends of the rods, which ends project slightly beyond the ends of the sleeves. The recesses in the front face of the pole piece 50 are shallower than would be needed to receive the end caps 79 and receive, instead, the projecting ends of the ceramic rods, which fit loosely in the recesses. When the electrode system has been aligned the flanges 108 are welded to the face of the pole piece. Similar sleeves 107 are provided in conjunction with the waveguide choke member 57, passing through clearance holes in the face portion, the flanges 107 being welded to sheets. One or more of the rods 53 is plated over the whole distance between the pole piece and the member 57, as indicated at 109, thus replacing the shorting bars 59. The same arrangement is provided for the members 54 and 58 at the other end of the travelling wave tube.

Before proceeding further with the assembly of the travelling wave tube it is necessary to seal the glass envelope 38 to the end cap 39 and to the end cap member 46 so that the members 64, 63 and 46 are coaxial and the correct distance apart. These parts are therefore assembled in a jig and the glass-metal seals effected; this is the only accurate glass-working operation called for in the manufacture of the envelope and is readily performed without demanding very close limits on the glass tubing used.

When the electrode assembly mounted on the electron gun structure has been aligned and welded up and the envelope 38 has been sealed to the end cap 39 and to the member 46 and provided with its resistive coating 106, the electrode assembly is slid into the envelope through the end cap member 46 and pushed home so that the member 63 projecting from the electron collector electrode seats in the mica washer 93. The end cap members 82 and 46 are then flash brazed together, as described earlier, and the travelling wave tube is processed. After processing the exhaust tubulation 65 is squeezed off and the cooler 43 is secured over the tube 64. As the cooler is a close fit over the tube 64 the projection 95 is accurately coaxial with the electrode system.

19 The bore of the cooler may be provided with a step, as indicated at to seat against the end of tube 64. It is also secured with the key way 99 in correct angular position with respect to the probe antennae 61.

For use the travelling wave tube, with cooler attached, is inserted through the cylinder 88 (Fig. 5) until the cooler engages with its seating in the boss 96 and is there held by means of the screw 101. The tube is then correctly positioned in all directions with respect to the assembly 86. If desired the assembly 86 may include a forced air cooling duct, indicated by the dotted line 111.

It will be seen that in the travelling wave tube of the present invention the use of the glass envelope to position the electrodes has been reduced to a minimum. The glass envelope can be made from standard glass tubing having ordinary commercial glass tolerances. The electrode assembly is rigidly supported whilst longitudinal expansion is permitted by the elasticity of the mica washer 93. The travelling wave tube may be made more easily and more accurately than that of Fig. l and is considerably shorted due to the substantial reduction of the inoperative part of the length of the electron beam. The tube is therefore more eflicient in use, due to the integral cooler it can handle more power, the magnetic focusing is not quite so critical and there is a considerable saving in the consumption of power required to provide the magnetic focusing field.

Although in Fig. 5 we have shown the travelling wave tube in association with electro-magnet focusing means it will be evident that it can also be used, if desired, with a permanent magnet focusing arrangement.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What we claim is:

l. A traveling wave tube comprising a first unitary assembly which comprises an electron gun, a terminal plate having leads hermetically sealed therethrough rigidly connected to said gun forming an electron gun structure, a waveguide choke inner sleeve member joined to said electron gun structure by a plurality of insulating rods, a helical coil for guiding electromagnetic waves to interchange energy with the electrons projected from said gun, a helix supporting tube of insulating material supported at its respective ends by the said electron gun structure and the said inner sleeve member, said helix being positioned within and supported by the said helix supporting tube, a first probe antenna joined to one end of the said helix and secured to the electron gun structure, a second probe antenna joined to the other end of the said helix and secured to the said inner sleeve member, two further waveguide choke inner sleeve members respectively secured from and opposing the electron gun structure and the first-mentioned said sleeve member, a metal member of predetermined diameter sealed to said terminal plate, an insulating plate provided with central opening rigidly supported by said first-named sleeve member at the end of said helical coil remote from said gun structure; and a second unitary assembly comprising an insulating envelope, and an electron collector electrode and a metal cap sealed to one end of said envelope, said collector electrode having a shoulder dimensioned to fit against said insulating plate of said wave guide choke member and a portion fitting into said central opening to provide correct longitudinal positioning and centering of said two assemblies, the other end of said envelope being sealed to said first named metal member.

2. A travelling wave tube according to claim 1, in which each of the said sleeve members comprises a plane face portion transverse the longitudinal axis of the travelling wave tube and a skirt portion projecting axially therefrom a quarter wave length at the mean operating frequency of the travelling wave tube.

3. A travelling wave tube according to claim '1 in which the said gun structure comprises a cylindrical ferromagnetic pole piece member surrounding a cathode and a beam forming electrode and is closed at the end facing the helix by a face portion integral with the electron gun anode.

4. A travelling Wave tube according to claim 3 in which said first mentioned waveguide choke inner sleeve member is made of ferromagnetic material.

5. A travelling wave tube according to claim 1, in which the said further waveguide choke members are secured respectively to the electron gun structure and to the said first mentioned waveguide choke member each by short-circuiting conducting means.

References Cited in the file of this patent UNITED STATES PATENTS Spooner Feb. 10, Pierce July 1, Hines Aug. 26, Mueller Dec. 1, Harman Mar. 16, Morton Oct. 19, Hines et al. Apr. 23, Molnar et 'al. June 25, Bianculli Sept. 10, Robertson Oct. 1,

FOREIGN PATENTS Great Britain July 2, 

